Internet Engineering Task Force P. M. Hallam-Baker
Internet-Draft Comodo Group Inc.
Intended status: Standards Track April 24, 2013
Expires: October 26, 2013
X.509v3 TLS Feature Extensiondraft-hallambaker-tlsfeature-02
Abstract
The purpose of the TLS Feature extension is to prevent downgrade
attacks that are not otherwise prevented by the TLS protocol. In
particular, the TLS Feature extension may be used to mandate support
for revocation checking features in the TLS protocol such as OCSP
stapling. Informing clients that an OCSP status response will always
be stapled permits an immediate failure in the case that the response
is not stapled. This in turn prevents a denial of service attack
that might otherwise be possible.
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Internet-Draft X.509v3 TLS Feature Extension April 2013
The purpose of the TLS Feature extension is to prevent downgrade
attacks that are not otherwise prevented by the TLS protocol.
Since the TLS protocol itself provides strong protection against most
forms of downgrade attack including downgrade attacks against cipher
suite choices offered and client credentials, the TLS Feature is only
relevant to the validation of TLS protocol credentials. In
particular to the revocation status of the server credentials
presented.
At the time of writing, the only TLS feature extensions that are
relevant to the revocation status of credentials is the Certificate
Status Request extension (status_request) Multiple Certificate Status
Extension (status_request_v2) These extensions are used to support
in-band exchange of OCSP tokens, otherwise known as OCSP stapling.
These extensions are described in [RFC6066] and [draft-pettersen-tls-ext-multiple-ocsp-03].
The OCSP stapling mechanism described in [RFC6066] permits a TLS
server to provide evidence of valid certificate status inband. When
this information is provided inband, the privacy, performance and
reliability concerns arising from the need to make a third party
connection during the TLS handshake are eliminated. A client cannot
however draw any conclusion from the absence of inband status
information unless it knows that the legitimate server would have
provided it. The status information might have been omitted because
the server does not support the extension or because the server is
witholding the information intentionally, knowing the certificate to
be invalid.
The inclusion of a TLS feature extension advertising the
status_request feature in the server end entity certificate permits a
client to fail immediately if the certificate status information is
not provided by the server. The need to query the OCSP responder is
eliminated entirely. This improves client efficiency and more
importantly prevents a denial of service attack against the client by
either blocking the OCSP response or mounting a denial of service
attack against the OCSP responder.
Since the TLS Feature extension is an option, it is not likely that
an attacker attempting to obtain a certificate through fraud will
choose to have a certificate issued with this extension. Such risks
are more approrpriately addressed by mechanisms such as Certificate
Authority Authorization DNS records RFC 6844 [RFC6844] that are
designed to prevent or mitigate mis-issue. Nevertheless a
Certification Authority MAY consider the presence or absence of a
required TLS feature as one factor in determining the level of
additional scruitiny a request should be subject to.
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Internet-Draft X.509v3 TLS Feature Extension April 2013
A server offering an end entity certificate with a TLS feature
extension MUST satisfy a client request for the specified feature
unless this would be redundant as described below. Otherwise clients
MAY refuse connection. It is important therefore that a
Certification Authority only issue certificates that specify features
that match the configuration of the server and that the server is
capable of verifying that its configuration is compatible with the
feature declaration of the certificates it offers. Ideally, the TLS
feature declaration would be specified by the certificate request
generator as part of the certificate issue process.
A client feature request is redundant if the purpose of the request
is fully satisfied by another feature. For example, a server need
not satisfy a client request for the status_request feature if the
status_request_v2 is offered and satisfied.
In the case that the cached_information feature is offered and
satisfied, a client request for the status_request or
status_request_v2 features is satisfied if and only if the cached
credentials referenced include the OCSP status information necessary
to establish the certificate status.
This document describes the use of the TLS feature in PKIX end entity
and certificate signing certificate and a mechanism that MAY be used
to describe support for the specified features in-band for the most
commonly used certificate registration protocol.
3. Syntax
The TLS Feature extension has the following format:
tls-feature OBJECT IDENTIFIER ::= { id-pe 1 }
Features ::= SEQUENCE OF INTEGER
The TLS Feature Extension SHOULD NOT be marked critical. RFC 5280
[RFC5280] requires that implementations that do not understand the
extension MUST reject the certificate. Marking the TLS Feature
Extension critical breaks backward compatibility and is not
recommended unless this is the desired behavior. Implementations
that process the extension MUST ignore the criticality bit setting.
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Internet-Draft X.509v3 TLS Feature Extension April 20133.1. TLS Feature
The TLS Feature extension lists a sequence of TLS extension
identifiers (features) that a TLS server compliant with the feature
declaration MUST support and satisfy on client request.
This specification does not require a TLS client to offer or support
any TLS feature regardless of whether it is specified in the server
certificate's TLS Feature extension or not. In particular a client
MAY request and a server MAY support any TLS extension regardless of
whether it is specified in a TLS Feature extension or not.
If a TLS Feature extension specifies a TLS feature, a server offering
the certificate MUST support the extension specified and MUST comply
with any specific requirements specified for that feature in this
document or in the document that specifies the TLS feature.
3.1.1. status_request
If the TLS status_request feature is specified in the TLS Feature
extension and a TLS client specifies the status_request feature in
the Client Hello, a server MUST return a valid OCSP token for the
specified server's End Entity certificate in the response.
3.2. Use3.2.1. Certificate Signing Request
If the certificate issue mechanism makes use of the PKCS#10
Certificate Signing Request (CSR) [RFC2986], the CSR MAY specify a
TLS Feature extension as a CSR attribute. A server or server
administration tool should only generate key signing requests that it
knows can be supported by the server for which the certificate is
intended.
3.2.2. Certificate Signing Certificate
When present in a Certificate Signing Certificate (i.e., CA
certificate with the key usage extension value set to keyCertSign),
the TLS Feature extension specifies a constraint on valid certificate
chains. Specifically, a certificate that is signed by a Certificate
Signing Certificate that contains a TLS Feature extension MUST
contain a TLS Feature extension which MUST offer the same set or a
superset of the features advertised in the signing certificate.
While relying parites (i.e., clients) MAY reject certificates that do
not comply with this requirement, the use of TLS Feature extension in
Certificate Signing Certificates is primarily intended for use by
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Internet-Draft X.509v3 TLS Feature Extension April 2013
parties seeking to evaluate the performance of certificate issuers
and MAY be ignored by clients.
3.2.3. End Entity Certificate
When specified in a server End Entity Certificate (i.e. a
certificate that specifies the id-kp-server EKU), the TLS Feature
extension specifies criteria that a server MUST meet to be compliant
with the feature declaration.
In the case that a client determines that the server configuration is
inconsistent with the specified feature declaration it MAY reject the
TLS configuration.
In the case that a client determines that the server configuration is
inconsistent with a feature declaration specifying support for the
TLS status_request extension it SHOULD reject the TLS configuration.
3.3. Processing3.3.1. Certification Authority
A CA SHOULD NOT issue certs with a TLS Feature extension unless there
is an affirmative statement to the effect that the end entity intends
to support the specified features. For example the use of a Feature
extension in the CSR or through an out of band communication.
3.3.2. Server
A TLS server certificate containing a TLS Feature extension MAY be
used with any TLS server that supports the specified features. It is
not necessary for the server to provide support for the TLS Feature
extension itself. Such support is nevertheless desirable as it can
reduce the risk of administrative error.
A server SHOULD verify that its configuration is compatible with the
TLS Feature extension expressed in a certificate it presents. A
server MAY override local configuration options if necessary to
ensure consistency but SHOULD inform the administrator whenever such
an inconsitency is discovered.
A server SHOULD support generation of the Feature extension in CSRs
if key generation is supported.
3.3.3. Client
A compliant client SHOULD reject a TLS connection with security
properties that are inconsistent with the specified TLS Feature
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Internet-Draft X.509v3 TLS Feature Extension April 2013
extension. A compliant client MAY accept such a TLS connection
request however if it is determined that doing so is appropriate in
particular circumstances.
4. Acknowledgements
[List of CABForum and PKIX contributors]
5. Security Considerations5.1. Alternative Certificates and Certificate Issuers
Use of the TLS Feature extension to mandate support for a particular
form of revocation checking is optional. This control can provide
protection in the case that a certificate with a TLS Feature is
compromised after issue but not in the case that the attacker obtains
an unmarked certificate from an issuer through fraud.
The TLS Feature extension is a post-issue security control. Such
risks can only be addressed by security controls that take effect
before issue.
5.2. Denial of Service
A certificate Issuer could issue a certificate that intentionally
specified a feature statement that they knew the server could not
support.
The risks of such refusal would appear to be negligible since a
Certificate Authority could equally refuse to issue the certificate.
5.3. Cipher Suite Downgrade Attack
The TLS Feature extension does not provide protection against a
cipher suite downgrade attack. This is left to the existing controls
in the TLS protocol itself.
6. IANA Considerations
No action by IANA is required.
7. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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